[TLS] PR#625: Change alert requirements

[Eric Rescorla <[e-mail]>]

PR: https://github.com/tlswg/tls13-spec/pull/625

Currently the TLS spec requires implementations to send alerts under various
fatal conditions. However, many stacks actually don't send alerts but
instead
just terminate the connection. Several people have argued that we should
relax
the requirement.

At the September 2015 interim there was consensus to instead encourage
sending alerts and require that if you send an alert, you send a specific
one.
I've finally gotten around to producing a PR that does this (link above).
This
PR:

- Harmonizes all the language around alert sending (though perhaps I missed
  a couple of places)
- Tries to make which alerts to send clearer in the alert descriptions to
avoid
  having to specify individually how to handle every decision.
- Relaxes the requirement as listed above.

Note that these are to some extent orthogonal changes; even if we decide to
continue mandating sending alerts, that should be listed in one location not
scattered around the spec.

I know that there wasn't universal consensus on relaxing the requirement to
send, so I'll await WG discussion and the chairs decision on how to handle
this PR.

-Ekr